Xeriscaping: Sources of New Native Ornamental Plants

Bart C. O'Brien

The use of the word xeriscape as a landscaping term has not received widespread
public acceptance, even though its precepts are sound and widely followed.
"Xeriscape" has often been interpreted by the general public to mean
"zero-scape" and was equated with severe minimalist drought tolerant plantings.
The terms "sustainable landscapes" and "appropriate horticulture" better convey
the full intent of "xeriscape." Native plant materials are a vital and viable
element of the future of xeriscape landscapes and gardens across America and
are the primary focus of this paper. As more gardens and landscapes are
designed and installed true to their local conditions, opportunities will
continue to expand for the use and development of new native plants for
ornamental horticulture.

The word xeriscape was coined in 1981, to capture the idea of water conserving
landscapes but conceptually xeriscape involves much more. There are seven
essential components of xeriscaping: appropriate planning and design;
appropriate/minimal turf areas; appropriate soil preparation and analysis;
appropriate plant selection; appropriate watering methods; appropriate use of
mulches; and appropriate landscape maintenance. In essence, the underlying
concept behind xeriscape is the doctrine of regionally appropriate
horticulture.

In practice, however, xeriscape often has been reduced to the landscaping idea
of maximizing the efficient use of water in gardens and landscapes by creating
areas which group plants by their water requirements. Typically this has been
interpreted as growing the plants with the greatest water need nearest the home
and transitioning to very low to no irrigation at the zone furthest from the
home. This zoning of the landscape according to water use may also have
additional benefits pertaining to fire breaks, wildlife value, and in enhancing
the diversity of plants that may be grown in the garden.

Due to the dominance of our primarily temperate European horticultural
heritage, there has always been a predisposition toward water-loving exotic
plants in our landscapes and gardens. The westward expansion and settling of
the country brought these same traditions and plants into increasingly dry and
warmer regions that are prone to long periods of drought. As long as water was
cheap and abundant there wasn't a problem and a more or less standard set of
coast-to-coast plants thrived in spacious green havens. Over much of the
country, water is the limiting resource governing the use of ornamental plants
in gardens. The huge increase in population in the U.S. Southwest has in many
areas outstripped local water supplies. In some areas of the country, water
for landscape or garden use is seasonally rationed or restricted. The cost of
water, even during years of abundant rainfall, is always increasing. Water
quality issues, including salinity and contamination by nitrates and bacteria,
lead to increased costs due to necessary water treatment programs. As time
passed, our use of gardens and landscapes changed from seasonal retreats and
visual frames for our homes and buildings to heavily used outside multi-purpose
spaces. These changes resulted in the establishment of the xeriscape concept.

There are a number of trends in home landscapes and gardens which will
influence the ornamental plant industry. Those which I consider to be among
the most important are: (1) the decrease in the amount of time, money and
expertise that most households have to invest in properly planting and
maintaining gardens; (2) the shrinking in average size of new gardens; (3) the
increasing use of gardens as living spaces and outdoor rooms; (4) the
increasing cost of water, labor, fertilizers, and chemicals; and (5) the
restriction or limiting of water use for garden and landscape purposes.

Although there is always a market for colorful annuals, biennials, and
short-lived perennials, these are generally more difficult groups to depend
upon in landscapes and gardens. Longer lived perennials, subshrubs, shrubs and
trees are the backbone of landscapes and gardens and will constitute the
primary nursery market.

Under current conditions, I am convinced that a greater diversity of plants
which flourish under existing climatic conditions across the country are
needed. These changes should be viewed as potential opportunities and require
the search for, and development of, perennials, subshrubs, shrubs and trees in
the following categories: dwarf or compact forms of existing plants; new plants
of small stature; plants that are all season performers; low maintenance
plants; long, or continuous, seasons of color, be it flowers, foliage, fruits,
stems or bark; good-looking, water-thrifty plants; and plants that attract
wildlife.

Native plants which fall into any or all of these categories will be
particularly desirable as they have the additional benefit of fitting into the
local and regional conditions both visually and ecologically. This opportunity
creates a new, relatively unexploited regional niche market for the nursery
industry, and should eventually lead to a more striking regionalism in the
American landscape vernacular.

There are a number of people and institutions throughout the country looking at
our native plants as a direct or secondary source of new introductions for the
nursery industry, but anyone can become involved in this process. The methods
employed to create new native ornamentals can be as informal as selection from
the wild or as sophisticated as plant breeding including even genetic
engineering.

The majority of these large genera and families have yet to be tested or
evaluated for horticultural purposes in a serious systematic fashion including
most perennial members of Phacelia, Monardella, Eriogonum,
and Astragalus. A number of these large genera have well known
propagation problems (such as commercially viable method for asexual
propagation of Quercus and Calochortus) or are known to be
especially difficult to grow and/or maintain under nursery conditions (like
Calochortus and many native Delphinium and Lupinus
species).

The selection of an individual plant for possible introduction is generally
accomplished by finding a desirable feature or set of features that are deemed
desirable. These individuals can be found in the wild or in the garden and
often represent extremes of the natural variation: compact growth habit,
different foliage, albinos, and unusual color forms. Additional factors such
as mutations including witch's brooms, variegated foliage, and natural hybrids
also contribute to the possible pool of plants to choose from. Traditionally
these matters of chance have been the most frequently reported sources of many
excellent cultivars: Heterotheca (Chrysopsis) villosa 'San
Bruno Mountain', a sterile, free-flowering dwarf selection; Artemisia
pycnocephala 'David's Choice', a heat tolerant compact selection;
Erigeron 'W.R.', a compact, free-flowering, heat and cold tolerant
chance wild hybrid between Erigeron glaucus and an unknown species; and
Acer macrophylla 'Seattle Sentinel', a fastigiate selection.

Another especially rich area for horticultural selection can be found in nearly
all taxonomically confusing groups at either the generic [example:
Arctostaphylos and Zauschneria (Epilobium)] or specific
[example: Mimulus aurantiacus or Mahonia (Berberis)
aquifolium] level. Taxonomic chaos is often indicative of the extreme
plasticity found naturally in the group and by inference the potential
horticultural availability of variation to select from. Similarly, there are a
number of extremely variable species that are nearly as ripe for selection as
the taxonomically complicated groups listed above. These species include
Rhamnus californica, Erigeron glaucus, Ceanothus
maritimus, Quercus chrysolepis, and Juniperus communis.

Peripheral populations of a desirable plant are another source of native plants
for landscapes and gardens. Plants from these populations may be more tolerant
of heat or cold, higher or lower elevations, drought or wetness, may exhibit
resistance to diseases or pests, or may be adaptable to a different soil type
or condition. An excellent example of this phenomena is Arctostaphylos
uva-ursi 'Point Reyes', a selection from the fog-bound headlands of the
Point Reyes peninsula in Marin County that is extremely tolerant of heat and
drought in comparison to typical members of the species. The small relictual
San Bernardino Mountains population of Populus tremuloides is remarkably
well adapted to growing conditions at low elevations in southern California.
When plants from these peripheral populations are not themselves
horticulturally desirable, they may carry genes which may be useful in a
breeding program.

Another particularly rich source of new plants to investigate are the genera
that have been dependably used in landscapes and gardens before, and look for a
different species with desirable characteristics. Cultivars of the "new"
species can be selected directly or the "new" species may be used to create
hybrids with the "established" species. The named Heuchera maxima x
Heuchera sanguinea hybrids: 'Genevieve', 'Opal', 'Santa Ana Cardinal',
'Susanna', and 'Wendy', created by Dr. Lee W. Lenz are excellent examples of
this approach.

Breeding programs involving controlled crosses and hybridization between
species are relatively rarely encountered in the native flora. There are,
however, a few exemplary programs which have yielded many fine plants: the
pacific coast hybrid irises, the Mimulus hybrids (of the section
Diplacus), the Lewisia cotyledon complex, and Lilium
hybrids.

All new cultivars should be named, described, published and registered with the
proper registration authority. A list of all currently accepted registration
authorities for ornamental plants is available from the American Association of
Botanical Gardens and Arboreta (AABGA).

All too frequently, a promising native plant species or cultivar does not live
up to its potential. Almost without exception, these plants are victims of
poor testing and evaluation.

Establishing a set of desirable traits and characteristics of plants which the
public actively wants or desires is a critical first step in the development of
a new plant introduction strategy. Consulting with focus groups from the
nursery industry, the gardening public, native plant enthusiasts, realtors,
horticultural groups and others should provide the researcher with plenty of
input on a variety of desirable plant traits. As progress is made in the
selection process, the new plants should be planted out in a number of test
sites. Test plantings should then be evaluated by a number of outside
reviewers. Plants surviving through the testing and evaluation process are
then ready to be introduced and marketed to potential user groups. The most
recent successful example of a western native plant to go through such a
thorough program is Arctostaphylos uva-ursi 'Vancouver Jade', an
introduction out of the University of British Columbia's well known plant
introduction program.

Everyone involved with the selection, use and promotion of new native plant
species and cultivars and/or any of their close relatives must be fully aware
of the potential negative consequences of their use in gardens and landscapes.
Most of these concerns center around the issue of conservation of genetic
resources and the genetic pollution of native plant populations in the wild.
Three examples from the California flora will serve to illuminate this issue.

Nevin's Barberry [Mahonia (Berberis) nevinii], an
extremely rare plant in nature, is found in widely distributed and ecologically
varied small populations in southern California. It is, however, a relatively
commonly seen landscape plant throughout the state. A molecular level study of
these plants showed that there is little to no variation present in the gene
pool. Therefore, the seed source of plants to be planted in the vicinity of
the remaining individuals in the wild is not of concern--there will be no
adverse consequences to the gene pool.

The California dandelion (Taraxacum californicum) from the wet meadows
in the San Bernardino Mountains is a rare plant whose continued existence is
threatened by ongoing hybridization with the common European dandelion
(Taraxacum officinale), a weedy pest plant that is common throughout the
range of the rare species.

The Monterey pine (Pinus radiata) is known from five geographically
isolated populations (three are from central coastal California and two are on
islands off Baja California). This tree is a significant forest tree in the
southern hemisphere, particularly in New Zealand. Material from all five
populations has been grown together on a massive scale in New Zealand, such
that the primary seed source of Monterey pine (even in California) is from New
Zealand. The issue of primary concern here is that the "mongrel" Monterey
pines of New Zealand mixed origin will genetically pollute the "pure" native
stands of these trees, such that the unique genetic character of the California
populations will be lost due to homogenization of the gene pool over time (the
Baja California populations are not threatened in this way at this time).

The development of new native plant crops for use in xeriscape landscapes and
gardens is still wide open with innumerable opportunities waiting for the
interested or inspired investigator. As more work is done to select and
develop the native flora for use in gardens, the more likely it will be that
these deserving plants will be used and appreciated by the general public.
When the public is aware and appreciative of the beauty and utility of native
plants they will be much more open to the conservation and preservation of this
essential component of our natural heritage. The single most important caveat
regarding the horticultural use of these native plants is that their use should
not be allowed to adversely impact native plant genetic resources.